In order to thermally fix a toner image transferred onto sheet, an image forming apparatus, such as a laser printer, is usually equipped with a fixing device having a heating roller and a press roller. During a period in which a sheet passes between the heating roller and the press roller, a toner image transferred on the sheet is thermally fixed.
As shown in FIG. 9, a heating roller 100 of the fixing device usually assumes a cylindrical shape and has a length responsive to the width of a sheet of the maximum size so that a sheet of the maximum size allowed by the image forming apparatus can be subjected to thermal fixing. One heater, formed from a halogen lamp B, is accommodated in the heating roller 100 in the axial direction thereof and over essentially the entire length thereof. The heating roller 100 is heated over essentially the entire length thereof in the axial direction.
In order to determine an anomaly in the temperature of the heating roller 100, a temperature detection unit 101, which utilizes a temperature detecting element, is provided on the surface of the center in the heating roller 100 in a noncontact manner.
However, when a small-size sheet 102a (of e.g., A6-size) and a large-size sheet 102b (of, e.g., A4-size) are subjected to thermal fixing by use of a single fixing device, the heating roller 100 is usually heated, by the heater, over essentially the entire axial length thereof corresponding to a width of the large-size sheet 102a. When the small-size sheet 102a is being subjected to thermal fixing, the sheet 102a draws heat from the surface of an area of the heating roller 100, which remains in contact with the small-size sheet 102a. A control unit 103 performs temperature control such that the temperature of the area, where the small-size sheet 102a contacts, is maintained within a given range. Therefore, the other area of the heating roller 100 where the small-size sheet 102a does not contact; i.e., an area of the heating roller 100 outside both ends of the small-size sheet 102a in the widthwise direction thereof, becomes higher in surface temperature than the center of the heating roller 100 in the widthwise direction thereof.
However, when the large-size sheet 102b is subjected to thermal fixing, the higher-temperature area (i.e., the area of the heating roller 100 outside both widthwise ends of the area where the small-size sheet 102a has contacted) contacts the large-size sheet 102b. When that area has been excessively heated, there arises a hot offset resulting from excessive fixing of toner (i.e., an offset caused as a result of excessively-fused toner adhering to the surface of the heating roller 100).
Even if the surface temperature of the areas of the heating roller 100 outside both widthwise ends of the small-size sheet 102a has become higher than the center surface temperature of the heating roller 100 when the small-size sheet 102a is thermally fixed, the control unit 103 fails to determine occurrence of an “anomaly,” sometimes resulting in fusing of the heating roller 100 or breakage of the fixing device. If the temperature of the heating roller 100 has risen excessively or the fixing device has become broken, high risks are posed to the user.
To prevent this problem, a conceivable method is to place a temperature detection unit at both axial ends of the heating roller in addition to disposing the temperature detection unit in the axial center of the heating roller and to detect the temperature of the heating roller by two temperature detection units.
For instance, there is described a technique pertaining to a fixing device including: a noncontact temperature detection unit provided on a heating roller in a noncontact manner within a sheet image formation area; a contact temperature detection unit provided on the heating roller in a contacted manner within a sheet non-image formation area; and a control unit for stopping supply of power to a heater when status determination unit for detecting a detected status of the noncontact temperature detection unit has determined occurrence of an anomaly in a temperature status. An anomaly is detected by the noncontact temperature detection unit on the basis of a temperature value of the heating roller detected by the noncontact temperature detection unit and a temperature value of the heating roller detected by the contact temperature detection unit (see JP-A-2004-126190).
By the technique such as that described in connection with JP-A-2004-126190, the detection status of the noncontact temperature detection unit can be rationally determined on the basis of the temperature values of the heating roller detected by the two temperature detection units. Occurrence of a problem, which would otherwise be attributed to an anomaly in the detection status of the noncontact temperature detection unit, can be prevented, and hence reliability can be enhanced.